In a wireless communication system, a base station communicates with a plurality of remote terminals, such as cellular mobile telephones. Frequency division multiple access (FDMA) and time division multiple access (TDMA) are the traditional multiple access schemes for delivering simultaneous services to a number of terminals. The basic idea underlying the FDMA and TDMA systems includes sharing the available resources so that several terminals can operate simultaneously without causing interference. The available resources that may be shared include frequencies and time intervals.
Telephones operating according to the GSM standard belong to the FDMA and TDMA systems in the sense that transmission and reception are performed at different frequencies and also at different time intervals. In contrast to the systems using frequency division or time division, the CDMA (Code Division Multiple Access) systems allow multiple users to share a common frequency and a common time channel by using coded modulation. Included among the CDMA systems are the CDMA 2000 system, the WCDMA system (Wide Band CDMA) and the IS-95 standard.
In CDMA systems, as is well known to the person skilled in the art, a scrambling code is associated with each base station and makes it possible to distinguish one base station from another. Furthermore, an orthogonal code, known by the person skilled in the art as an OVSF code, is allotted to each remote terminal, such as a cellular mobile telephone, for example. All the OVSF codes are mutually orthogonal, thus making it possible to distinguish one remote terminal from another.
Before sending a signal over the transmission channel to a remote terminal, the signal has been scrambled and spread by the base station using the scrambling code of the base station and the OVSF code of the remote terminal. In CDMA systems, it is again possible to distinguish between those which use a distinct frequency for transmission and reception (CDMA-FDD system), and those which use a common frequency for transmission and reception but distinct time domains for transmission and reception (CDMA-TDD system).